You cannot do this directly via PicoScope. There is a way round this, if you choose copy as text and go into Excel, past as a link, this will give you live readings. Channel B results will be under channel A. If you then input a formula to subtract one from the other and plot a graph it will give you a plot of the sum.

You have to be careful though, if you change your trigger point or change your time base and any other settings then your graph may turn out to be incorrect.